In electrostatographic imaging and recording processes such as electrographic reproduction, an electrographic reproduction apparatus is utilized to form an electrostatic latent image on a primary image-forming member such as a dielectric surface and is developed with a thermoplastic toner powder to form a visible image. The visible thermoplastic toner powder image is thereafter transferred to a receiver, e.g., a sheet of paper or plastic, and the visible thermoplastic toner powder image is subsequently fused to the receiver in a fusing station using heat or pressure, or both heat and pressure.
In the reproduction apparatus, various components, such as the primary image-forming member, the transport for the receivers, and/or the fuser may include a belt movable about a closed loop path. In each instance where a belt moving about a closed loop path is used, it is important that the particular belt be accurately controlled in the movement about the closed loop path such that it is steered for proper interrelation with other components of the reproduction apparatus as a reproduction is formed. Often a section of the circumference of the closed loop transport path, or a run of the path between path-defining rollers, must be held substantially constant in the location both axially and vertically to assure desired registration between apparatus elements. Since a most desirable, compact system uses a minimum of support/steering rollers, a sever requirement is placed on the steering method in order to satisfy the physical location consistency and accuracy. Motion of the belt in a direction perpendicular to the desired transport direction must be minimized to assure registration between the apparatus components in order to form a desirable image reproduction.
In the past it was well known to track belt movement in a direction perpendicular to the transport direction by a common belt edge sensor having an array of optical sensors. The belt edge position is digitally deduced by determining which optical sensors of the array are (or are not) blocked by the belt as it is transported about the closed loop path. The overall resolution of the belt edge sensor is determined by the spacing between the optical sensors in the sensor array. As such, the ability to accurately steer the belt is dependent upon the finite relationship of optical sensor and their spacing.